Some internal combustion engines such as gasoline engines and diesel engines are equipped with turbochargers. A turbocharger rotates a turbine through the use of a pressure of exhaust gas from an engine, and drives a compressor with the aid of a rotational force of the turbine to supercharge the engine. An engine thus equipped with a turbocharger may be provided with a bypass line through which an outlet passage of a compressor and an inlet passage of a turbine communicate with each other (e.g., Patent Document 1).
Engines provided with the bypass line are designed to efficiently driving an exhaust gas recirculation (EGR) system. The EGR system reduces the oxygen concentration in an intake air by partly circulating the exhaust gas to the intake air of the engine and lowers the combustion temperature of the engine in order to suppress the generation of nitrogen oxides (NOx) in exhaust gas. Engines provided with the EGR system are designed so that the emission rate of NOx is observed and exhaust gas recirculation is made to take place when the emission rate becomes higher than a predetermined level. If the intake pressure of the engine is higher than the exhaust pressure and exhaust gas hardly flows to the intake side, the bypass circuit is opened to allow the intake air to partly flow into the exhaust passage and reduce the intake pressure in order to facilitate the recirculation of exhaust gas. With this control arrangement, it is possible to efficiently conduct the EGR.
[Patent Document 1] JP 2001-165000 A (9-10, FIG. 1)